Syringes prefilled with a liquid drug (prefilled syringes) are used as medical syringes. In recent years, the prefilled syringes are increasingly used, because the prefilled syringes can be easily handled without the need for transferring a liquid drug into the syringe and can prevent medical malpractice such as transfer of a wrong liquid drug into the syringe.
Unlike conventional syringes (into which a liquid drug sucked up from a vial or other container is transferred immediately before use), the prefilled syringes are required to serve as a container which is kept in contact with the liquid drug for a long period of time.
A gasket to be used for such a syringe is generally made of a crosslinked rubber. It is known that the crosslinked rubber typically contains various crosslinking components, and these crosslinking components and their thermally decomposed products are liable to migrate into the liquid drug when the liquid drug is kept in contact with the gasket. It is also known that these migrating components adversely influence the efficacy and the stability of some liquid drug.
When the prefilled syringe is used, the gasket is required to smoothly slide with respect to a syringe barrel of the syringe. In general, the gasket made of the crosslinked rubber cannot be used because of its poorer slidability. Therefore, it is a general practice to apply a silicone oil onto a surface of the barrel or the gasket. However, it is known that the silicone oil adversely influences the efficacy and the stability of some liquid drug.
From this viewpoint, a so-called laminated gasket product is known, which includes a rubber gasket body having a surface laminated with a highly slidable film. By covering the surface of the rubber gasket body with the film, the components of the crosslinked rubber are prevented from migrating into the liquid drug. Further, sufficient slidability is ensured by laminating the surface with the highly slidable film.
Exemplary materials for the film to be used for these purposes include ultrahigh molecular weight polyethylenes and fluororesins which are highly slidable. Of these materials, the fluororesins are preferred because of their high slidability and chemical stability. Of these fluororesins, polytetrafluoroethylenes (PTFE) are particularly preferred because of their very high slidability and stability.